Desalination process

ABSTRACT

Brackish water is desalinated by contacting it with one or more ion exchange resins followed by reverse osmosis. Brine rejected in reverse osmosis treatment can be used to regenerate a used ion exchange resin.

O United States Patent 1151 3,639,231 Bresler 51 Feb. 1, 1972 [54] DESALINATION PROCESS [56] References Cited [72] Inventor: Sidney A. Bresler, New York, N.Y. UNITED STATES PATENTS 1 Assienee Bresler and Associates New York, 3,526,320 9/1970 Kryzer ..2|/32| x NY. 2,606,870 8/1952 Pembenon e1:a1..... ....210/26 2 l d Nov. 13, Beohner [2]] Appl. No.: 89,429 Primary Examiner-Samin N.Zaharna Attorney--Flynn & Frishauf 7 AB RA T [52] U.S.Cl ..2l0/23,210/30,210/259 1 ST C V 51 Int. Cl. ....B01d 13/00, B0ld /04 Brackish water is desalinared y contacting it with one or [58] Field of Search ..210/22, 23, 26, 30, 37, 38, mor i n ex hang r sins f llowe by reverse osmosis. Brine 210/259, 321 rejected in reverse osmosis treatment can be used to regenerate a used ion exchange resin.

12 Claims, 3 Drawing Figures /2 V4 /6 /a /0 /7 l9 l 2 L 2 2 i.l 1 j DESALINATION PROCESS FIELD OF INVENTION This invention has to do with a process for desalting a saltcontaining water such as a brackish water.

BACKGROUND OF THE INVENTION Various processes may be used to reduce the dissolved solids contents of a salt-containing water to the level at which the water is suitable for drinking or for industrial use such as cooling tower makeup. Such processes include ion exchange, reverse osmosis, electrodialysis, evaporation (multiple-effect or multiple-staging), vacuum freeze, and vapor compression. The first three of these processes may be most suitable for treating a brackish water (containing less than 10,000 parts per million of total dissolved solids), while the others may be most suitable for treating more concentrated solutions such as sea water (33,000 ppm.

Although not the optimum process for desalting brackish water in all instances, the reverse osmosis method offers certain advantages. Reverse osmosis involves the use of a semipermeable membrane which, under the application of pressure to a feed stream, permits passage of water molecules but holds back all or part of the undesired solute in the feed stream. Unlike ion exchange, the cost of chemicals for regeneration is minimized. Unlike electrodialysis, reverse osmosis does not require relatively large amounts of direct-current electric power and its effectiveness is not as sensitive to the temperature of the feedwater.

In any desalination process, the brackish feedwater is brought to a desalination plant, where it is separated into essentially two streams. One stream consists of product water, which is relatively pure. The second, or waste, stream contains the bulk of the dissolved solids. It is usually desired that this waste stream be as concentrated as possible, for three reasons. First, the disposition of such a waste stream frequently is costly, with the cost being related more to the quantity of liquids to be disposed of rather than to the amount of dissolved material. Furthermore, the means for such disposal, such as the introduction of the waste stream into a deep aquifer, may be limited. Second, the total supply of brackish water may be limited. If the dissolved solids can be concentrated into a smaller stream, the amount of resultant demineralized water will be increased. Third, this waste stream is a potential source of valuable salts and minerals. As

the concentration of this waste stream is increased, the cost of mineral recovery is decreased.

There is a limit to the degree of concentration of dissolved solids which can be produced in the waste stream. If this limit is exceeded, the concentrated solids will precipitate out of the solution. This precipitated material will tend to deposit on a transfer surface which may be employed in the desalination process, such as a reverse osmosis membrane or an ion exchange resin bead, decreasing the material transfer across this surface and increasing the amount of energy required to effect the separation of water from dissolved salts.

When demineralizing most brackish waters, the first salts to precipitate from the concentrated solution are compounds of calcium sulfate or calcium carbonate. If magnesium is present, magnesium hydroxide may form. Other salts such as sodium sulfate, sodium carbonate, calcium chloride and sodium chloride are much more soluble; as are the mineral acids. Under certain conditions, complex compounds of CaSO and Na SO may precipitate.

Two measures which are frequently employed to minimize or to prevent the fouling of the transfer surface by precipitated salts consists of lowering the pH of the solution and adding antiprecipitants. The addition of sufficient amounts of acid will inhibit the formation of calcium carbonate and magnesium hydroxide, and also of iron and aluminum compounds. The calcium sulfate solubility also may be effected. The addition of sodium hexametaphosphate, an organic polyelectrolyte, or similar compounds may inhibit the formation of precipitates and reduce the tendency of those which do form to become attached to the transfer surface. However, these measures are costly; and the efficacy of antiprecipitant usage is not altogether certain.

In general, precipitates and scale will form in a reverse osmosis unit before the saturation level is reached in the bulk liquid stream. This will take place because a salt concentration gradient will be established in what may be considered a boundary layer immediately adjacent to the semipermeable membrane. The thickness" of the layer, and the degree of salt concentration, is a function of many factors including the physical configuration of the reverse osmosis equipment and also the product flux and the Reynolds number of the bulk fluid. Salt concentrations in this boundary layer may be perhaps twice that in the bulk liquid.

The problem tends to become more severe as membranes are devised which enable one to obtain a higher water flux.

Reducing the boundary layer thickness by increasing the turbulent flow velocity in the bulk liquid stream or introducing turbulance-inducing devices, tends to create an increased pressure drop and increased power costs. Reducing the boundary layer thickness by reducing the flow channel results in increased equipment cost.

It is apparent that the concentration of the waste stream could be increased if changes could be made in the ionic composition of the inlet brackish water-for example, if calcium and/or magnesium radicals were replaced by sodium and/or hydrogen radicals; or if sulfate and/or carbonate radicals were replaced by chloride radicals; or if the least soluble ionic constituents were removed from the inlet water.

Certain ionic constituents can be removed by ion exchange or by lime-soda softening. The disadvantage of the former process is the cost of chemicals needed to regenerate the ion exchange resins. The disadvantage of the latter process is the cost of chemicals which are required (lime and/or soda ash) and the cost of the precipitator and other equipment.

SUMMARY OF THE INVENTION In accordance with this invention, it has been found that inorganic salts can be removed from an aqueous solution, particularly a brackish water, by contacting the solution with one or more ion exchange resins to remove certain ions from said solution, forcing the resulting solution through a reverse osmosis membrane which rejects salts present in said solution, collecting an aqueous product having an inorganic salt content substantially less than said aqueous solution and an inorganic salt product. The latter product can be used to regenerate the ion exchange resin.

DRAWINGS FIGS. 1 through 3 contain flow diagrams illustrating several embodiments of the invention.

SPECIFIC EMBODIMENTS OF THE INVENTION One technique for removing salts from a brackish water is illustrated by FIG. 1 and involves passing water in line 10 over a bed of a weakly acidic ion exchange resin in vessel 11. Typical of such resins are cross-linked acrylic resins which contain functional carboxylic acid groups and which operate at a pH range of from about 5 to about 14. Examples include Rohm and Haas IRC-84 and Diamond slhamrocks CC-3. Resin capacity ranges from approximately 5 to 70 kilograins per cubic foot, as CaCO in the process. of this invention. Bicarbonate present in the brackish water is converted to carbon dioxide and water, and an equivalent amount of calcium and magnesium ions are absorbed on the resin. If the equivalent amount of bicarbonate exceeds that of calcium, magnesium and other divalent or trivalent cations, then some monovalent cations will also be absorbed by the resin.

The effluent from the bed of weakly acidic ion exchange resin is then removed from vessel 11 through line 12 and is passed to degassing vessel 13 such as a tower packed with Raschig rings or the like. Carbon dioxide is removed through line 14. Degassed effluent is passed through line to vessel 16 wherein it is passed over a bed of a strongly acidic ion exchange resin in cation form, particularly in the form of a sodium cation. Typical of such resins are sodium sulfonates of styrene and divinyl benzene copolymer resins which operate within approximate pH limits of l to 14. Illustrative examples include Rohm and Haas Amberlite 200 and Diamond Shamrocks C-20. Resin capacity ranges from approximately 8 to 30 kilograins per cubic foot, as CaCO in the processes of this invention. Sodium regenerant requirements range from about 1 15 percent to about 300 percent of stoichiometric.

Substantially all of the remaining calcium and magnesium ions present in said effluent are absorbed on the strongly acidic ion exchange resin, and an equivalent amount of sodium ions are released into the resulting aqueous solution.

All or a portion of this softened and partially deionized water is removed from vessel 16 through line 17 and is passed to reverse osmosis unit 18, wherein it is further deionized. This unit contains a semipermeable membrane such as Gulf General Atomics spiral wound units or American Standards tubular units, which contain cellulose acetate membranes, or duPonts hollow, fine nylon fiber units. Nylon is a superpolymeric amide prepared from aliphatic dicarboxylic acids, such as adipic and sebacic acids and aliphatic diamines such as hexamethylene diamine. Other types of semipermeable membranes and other mechanical configurations also may be employed. Power requirements for such a unit range between about 6 and about I l kilowatt hours (kw.-hr.) per 1,000 gallons of deionized water.

The deionized water from the reverse osmosis unit or units can be utilized for municipal or industrial purposes as it is delivered therefrom through line 19. Alternatively, if product water specifications so permit, all or a portion of the deionized water (in line 19) from such a unit can be passed through line 20 and blended with the softened and partially deionized water taken from the strongly acidic ion exchange resin bed through line 21.

The concentrated salt solution or brine rejected by the membrane or membranes in a reverse osmosis unit is removed from vessel 18 through line 22 and is passed to unit 16 wherein it is used to regenerate the strongly acidic resin bed therein. A salt such as makeup sodium chloride in line 23 can be added to the concentrated salt solution as an aid in regenerating the resin. In order to minimize sodium regenerant requirements, the strongly acidic resin bed may be regenerated in an area of relatively low capacity but high regenerant efficiency. The regenerant stream removed via line 24 from the resin bed, after contact with the brine, is considered a waste stream, which is disposed of as by injection into a deep aquifer, evaporation system, etc. However, this stream can be processed further for recovery of its constituents.

The weakly acidic ion exchange resin in vessel 11 can be regenerated with sulfuric or other mineral acids in line 25, using approximately 10 percent more acid than stoichiometrically required, as is well known in the art. Regenerant stream is removed from vessel 11 through line 26.

If desired a portion of the water charge can be passed through line 27, or water from any one or more of lines l2, l5, l7 and I9, and combined with brine in line 22 for diluting the concentrated brine.

Therefore, by utilizing an otherwise waste brine stream to regenerate a resin bed, and by using a relatively small amount of acid to regenerate a second resin bed, the cost of extraneous chemicals is reduced substantially.

Modifications of the process described above are contemplated. For example, when the brackish water to be processed contains only a relatively small amount of bicarbonate ion, the initial bed of weakly acidic ion exchange resin in-vessel 11 can be omitted. Also when economic considerations dictate, this bed is omitted.

Another modification shown in FIG. 2 involves passing the effluent from the initial bed of weakly acidic ion exchange resin in vessel 11 over a bed of strongly acidic ion exchange resin in the acid form in vessel 16. Typical of the latter resins are Amberlite 200 and C-20, mentioned above. Resin capacity ranges from approximately 5 to 35 kilograins per cubic foot, as CaCO in this process. Acid regenerant requirements range from about 1 15 percent to about 300 percent of stoichiometric, acid being added through line 23. Substantially all of the remaining cations such as calcium, magnesium and sodium, are absorbed on the resin bed in vessel 16, and an equivalent amount of hydrogen ions are released into the solution in line 17. In this process, the reverse osmosis membrane which is employed in vessel 18 is capable of operating in an acid environment; a cellulose acetate of Gulf General Atomic, mentioned above, is suitable for this purpose.

In FIG. 2, the unused acid in line 24 can be passed to vessel 11 to regenerate the weakly acidic ion exchange resin therein. Additional acid can be added to line 24 (not shown). Waste stream 28 is removed from vessel 11 through line 28.

Still another modification involves the use of an anion exchange resin bed in place of a weakly acidic and/or strongly acidic resin bed to remove calcium and magnesium ions. Sulfate ions present in the brackish charge water are absorbed on the anion resin bed and an equivalent amount of chloride ions are released into the resulting solution. This is illustrated in FIG. 3, wherein an anion exchange resin is employed in vessel l6.

Illustrative examples of such resins are Rohm and Haas IRA-47 and Diamond Shamrocks ES-104. The former is a condensation polymer of epichlorohydrin and a polyamine mixture of diethylene triamine and triethylene tetramine. During the regeneration phase, the pH of solution passed to this resin bed is maintained at about 2-3 by the addition of hydrochloric acid through line 23. The capacity of the resin is approximately 20 kilograins per cubic foot, as CaCO and the regenerant requirement is approximately 7 pounds of salt per cubic foot which is charged via line 29.

Resin ES-l04 is a strongly basic resin comprising a polystyrene resin containing quaternary ammonium groups. The resin is used at pH values of l to 14 and at temperatures below about F. Approximately 10 percent of caustic soda is added to the sodium chloride regenerant solution used in regenerating this resin. The capacity of the resin is approximately 16 kilograins per cubic foot, as CaCO and the regenerant requirement is about 6 pounds of NaCl and about 0.4 pound of NaOH per cubic foot.

Rather than utilizing weakly acidic and/or strongly acidic resin beds to remove calcium and magnesium, a strongly basic anion resin bed can be employed. In such case, sulfate and other ions are absorbed on the resin bed and an equivalent amount of hydroxyl ions are released into the solution. Examples of such resins are Rohm and Haas IRA-40 and Nalco Chemicals Dowex SBR. Each is a styrene and divinyl styrene copolymer resin containing quaternary ammonium groups, and is useful at a pH range of 0 to 14. Resin capacity ranges from approximately 5 to 15 kilograins per cubic foot, as CaCO Regenerant requirements range from 1 to l0 pounds of NaOH per cubic foot. In this modification, the reverse osmosis membrane employed is one capable of operating in a basic environment. The nylon membrane of duPont, mentioned above, is suitable.

It is also contemplated that the processing techniques described above can be combined. For example, a portion of the feedwater can be treated by the process illustrated by FIG. 1 and another portion by the process shown by FIG. 2. Product and partially deionized water streams, with the optional addition of untreated feedwater, can be combined to produce water streams having the desired quality.

Ion exchange beds containing the same resin can be used in parallel or in series. Ion exchange can be conducted in fixed or moving beds, and include discontinuous, continuous, or other modes.

Regardless of the composition of the ion exchange bed, the waste or brine stream from a reverse osmosis unit can be used entirely or in part, to regenerate one of the ion exchange beds.

in those instances in which an insufficient amount of chemical regenerant is contained in the waste stream from the reverse osmosis unit, this stream is enriched by the addition of the necessary amounts of chemicals, such as sodium chloride,

EXAMPLE 1 This example is shown by FIG. ll. One million gallons per day (l MUD) of brackish water is charged through line M13. The composition of this water is shown in table 1 with sulfuric or other acids, or caustic soda or other alltali, as in refflgnce to m y The quantities and Compositions f dlcatefi abovestreams Elli, 1'7 and 19 are also shown.

It mlght pp that cohcehhahoh of h lh the effluent The strongly acid resin in vessel 16 is in the sodium form. regehel'aht stream from the h reslh bed would be Analyses given are approximate. Product water recovery is no greater Could he Obtained m the bum Stream a 10 90 percent. it may vary from about 50 percent to about 95 h Psmosls Plant, as @1 bound y the Same h l percent. Salt rejection is about 95 percent; it may vary from equilibrium. However, this is not correct, as the mechanism of about 50 percent {0 about 95 percent f monovalem ions fixchahge 1 1/ the use of f enables one to Obit-41h such as chlorides, sodium, potassium, nitrate, and bicars1gn1ficantly higher sulfate concentrations, or concentration of 1 m d f about 79 percent to over 99 Percent f other p Flp the flhal efflueht- Several Procedures divalentions such as calcium,rnagnesium and sulfate. may be ut lized, such as. EXAMPLE 2 a. Rapid regeneration, with a supersaturated regenerant being removed from the resin bed before sufficient time This l P 1 9 mustrawd by 'F'" the has elapsed for precipitation to Dawn strongly acidic resm 1s 1n the acid form, such as Amberhte 200 Staged regeneration, with a portion of the calcium m vessel 1h. Atllso, product water in lme 19 is blended in line other) ions removed by a dilute regcnerant stream and M walfmm wflmm vessel the balance by a more Concentrated Stream Sulfate 1011 present 1n the product 111 line 19 can be c.Countercurrent regeneration, with the most concentrated elchahged for Chloride 1 H ypassing all or part of this calcium (or other) ions removed by the concentrated 2' another res! bed such as [RA-47 or E5404 regenerant stream just before it leaves the ion exchange memmne f bed The chloride 1011 and sulfate ion values can be reduced Thfise methods are We known in the an and are not materially by operating reverse osmosis unit 11% to maintain a dascribfid further. percent product recovery rather than about percent as In some instances as illustrated in F168. 1&1, amounts of shown Alternatively product mm m can be passed fcedwater or water from one or more of neg m w H7 and 30 through asecond reverse osmosis unit to reduce such values.

w, 5 19, may be used to dilute the concentrated brine stream from Results pmvlded m table the reverse osmosis unit, before it is used as a regenerant EXAMPLEIi stream. This example is illustrated by F161. 2, with results being pro- Temperatures and pressures employed are not critical. r vided in table m Suitable temperatures and pressures are: 60-l20 l and at- 33 EXAM? E 4 mospheric to 150 p.s.i.a. L

The invention is illustrated further by the following typical This example is illustrated by Fit]. 3. Results are given in examples. table 1V.

TABLE 1 Line Flow, 11145.11 1. 1. 0 U 0 0 1 0.1) 0.1

(32100 1011 (221C011 10 C2100; 10 C0003 1011 (311003 CaCO; 1011 "mm dissolved S011 M 1. 8 .541 12.300 345 11.001

=befor0 line 27. =bel0r0 line 21. =1'ron1 vessel 18. :betore line 20.

TABLE II Line 10a 15 17 22 10 24 20 (Blended) Flow, 111. 111 1.0 1.0 1.0 0. 07a 0. 07s 0. 075 0.025

CaCOi Ion CaCO Inn 02100; 1011 (321003 Ion C3003 Ion CaCOi Ion (111003 1011 P0011 tntimis 1, 400 1, 300 1, 300 10, 420 203 10, 350 504 llullg 1 00 1 V ,0 1,200 850 1,200 050 1,200 860 0, 620 6,800 102 370 so. 100 00 100 06 100 06 800 70s 22 21 41 30 Total unions 1, 400 1, 300 1, 300 10, 420 293 564 Total dissolved solids... 1, 640 1,508 1, 639 12,300 347 667 =before line 20.

TABLE III Line Flow, 1114:.(1 1.0 1.0 1.0 0.1 0.9

01100; [on CaCOa Ion CaCOa Ion CaCO; Ion CaCOa Ion Total anions 1, 400 1, 300 1, 300 10, 377 291 Total dissolved solids. 1, 670 1, 508 999 7, 975 224 =bel'ore line 27. =before line 21. =irom vessel 18. =lieioro line 20.

TABLE IV Line * v lli 15 i v 17 22 19" Flow, n1.g.d 1.0 v 7-" 1.0 1.0 0.1 0.9

02100; Ion CaCOa Ion CnCO; Ion @5003 Ion CaCOz Ion Composition (p.p.m.):

0a.. 300 120 200 80 200 80 l, 598 639 45 18 Mg. 100 24 100 24 100 24 799 191 22 5 Na 1, 000 45s 1, 000 45s 1, 000 45s 7, 990 3,660 252 103 H 160 3 4 Total oati0ns I400 1, 300 1, 320 10, 547 296 Total dissolved solids.

l before line 27. before line 21. =Iron1 vessel 18.

What is claimed is:

l. The process for removing inorganic salts from an aqueous solution containing the same, which comprises:

a. contacting said solution with an ion exchange resin to remove ions present in said solution,

b. forcing the solution obtained in (a) through a reverse osmosis membrane which rejects and concentrates salts present in said solution,

c. collecting an aqueous product having an inorganic salt content substantially less than said aqueous solution, and an inorganic salt product, and

d. regenerating said ion exchange resin of (a) with said inorganic salt product collected in (c).

2. The process for removing inorganic salts from an aqueous solution containing the same, which comprises:

a. contacting said solution with an ion exchange resin active at a pH of from about 5 to about l4, to remove ions present in said solution,

b. contacting the solution obtained in (a) with an ion exchange resin active at a pH of from about I to about 14, to remove ions present in said last-mentioned solution,

c. forcing the solution obtained in (b) through a reverse osmosis membrane which rejects and concentrates salts present in said solution,

d. collecting an aqueous product having an inorganic salt content substantially less than said aqueous solution, and an inorganic salt product, and

e. regenerating said ion exchange resin of (b) with said inorganic salt product collected in (d).

3. The process of claim 2, wherein the aqueous solution is a brackish water.

4. The process of claim 2, wherein the ion exchange resin of (a) is an acrylic acid resin having carboxylic acid groups.

=bol'ore line 20.

5. The process of claim 2, wherein the ion exchange resin of (b) is a sodium sulfonate vin l benzene.

6. The process of claim wherein the membrane of (0) comprises cellulose acetate.

7. The process of claim 2, wherein the membrane of (c) comprises a nylon.

8. The process of claim 2, wherein the ion exchange resin of (a) is regenerated with the stream obtained by regenerating said ion exchange resin of (b) with said inorganic salt product.

9. The process of claim 2, wherein the ion exchange resin of (b) is a vinyl benzene containing a quaternary ammonium substituent.

10. The process for removing inorganic salts from an aqueous solution containing the same, which comprises:

a. contacting said solution with an ion exchange resin active at a pH of from about 5 to about 14, to remove ions present in said solution,

b. contacting the solution obtained in (a) with an ion exchange resin active at a pH of from about 1 to about 14, to remove ions present in said last-mentioned solution,

c. forcing the solution obtained in (b) through a reverse osmosis membrane which rejects salts present in said solution, and

d. collecting an aqueous product having an inorganic salt content substantially less than said aqueous solution, and an inorganic salt product.

1 l. The process of claim 1 wherein said inorganic salt product collected in (c) contains an acid component and wherein said ion exchange resin of (a) is regenerated with said acid component.

12. The process of claim 2 wherein said inorganic salt product collected in (d) contains an acid component and wherein said ion exchange resin of (b) is regenerated with said acid component. 

2. The process for removing inorganic salts from an aqueous solution containing the same, which comprises: a. contacting said solution with an ion exchange resin active at a pH of from about 5 to about 14, to remove ions present in said solution, b. contacting the solution obtained in (a) with an ion exchange resin active at a pH of from about 1 to about 14, to remove ions present in said last-mentioned solution, c. forcing the solution obtained in (b) through a reverse osmosis membrane which rejects and concentrates salts present in said solution, d. collecting an aqueous product having an inorganic salt content substantially less than said aqueous solution, and an inorganic salt product, and e. regenerating said ion exchange resin of (b) with said inorganic salt product collected in (d).
 3. The process of claim 2, wherein the aqueous solution is a brackish water.
 4. The process of claim 2, wherein the ion exchange resin of (a) is an acrylic acid resin having carboxylic acid groups.
 5. The process of claim 2, wherein the ion exchange resin of (b) is a sodium sulfonate vinyl benzene.
 6. The process of claim 2, wherein the membrane of (c) comprises cellulose acetate.
 7. The process of claim 2, wherein the membrane of (c) comprises a nylon.
 8. The process of claim 2, wherein the ion exchange resin of (a) is regenerated with the stream obtained by regenerating said ion exchange resin of (b) with said inorganic salt product.
 9. The process of claim 2, wherein the ion exchange resin of (b) is a vinyl benzene containing a quaternary ammonium substituent.
 10. The process for removing inorganic salts from an aqueous solution containing the same, which comprises: a. contacting said solution with an ion exchange resin active at a pH of from about 5 to about 14, to remove ions present iN said solution, b. contacting the solution obtained in (a) with an ion exchange resin active at a pH of from about 1 to about 14, to remove ions present in said last-mentioned solution, c. forcing the solution obtained in (b) through a reverse osmosis membrane which rejects salts present in said solution, and d. collecting an aqueous product having an inorganic salt content substantially less than said aqueous solution, and an inorganic salt product.
 11. The process of claim 1 wherein said inorganic salt product collected in (c) contains an acid component and wherein said ion exchange resin of (a) is regenerated with said acid component.
 12. The process of claim 2 wherein said inorganic salt product collected in (d) contains an acid component and wherein said ion exchange resin of (b) is regenerated with said acid component. 